1. Field of the Invention
This invention relates to a transmitter and to a control method for a transmitter, and in particular relates to a transmitter having a bus driving circuit which outputs differential signals, and to a control method for such a circuit.
2. Description of Related Art
In automotive control technology in recent years, numerous CAN (Controller Area Networks) have been used as networks connecting the various portions of automobiles. A CAN performs data communication using differential signals on a pair of signal lines. A CAN conforms to serial bus communication standards, and signals output to signal lines are generated by transceivers. One example of such a transmitter is disclosed in National Publication of Translated Version No. 2002-509682.
Here, a circuit diagram for a general transmitter 101 used in a CAN appears in FIG. 9. As shown in FIG. 9, the transmitter 101 has a PMOS transistor Tr1 (below simply called “transistor Tr1”), an NMOS transistor Tr2 (below simply called “transistor Tr2”), reverse-current prevention elements (diodes D1 and D2), and a control circuit 103. The transmitter 101 also has a power supply terminal VDD, ground terminal GND, and output terminals CANH and CANL.
The transistor Tr1 and diode D1 are connected in series between the power supply terminal VDD and output terminal CANH. A control signal CTRL1 of control circuit 103 is input to the gate of transistor Tr1. The anode of diode D1 is connected to the drain of transistor Tr1, and the cathode is connected to output terminal CANH. On the other hand, the transistor Tr2 and diode D2 are connected in series between the ground terminal GND and output terminal CANL. A control signal CTRL2 of control circuit 103 is input to the gate of transistor Tr2. The anode of diode D2 is connected to output terminal CANH, and the cathode is connected to the drain of transistor Tr2. The output terminal CANH and output terminal CANL are connected via a load resistance RL.
The transmitter 101 controls transistors Tr1 and Tr2, by the control signals CTRL1 and CTRL2 output by control circuit 103, to pass a current through the load resistance RL. By this current, a potential difference is induced across output terminals CANH and CANL, so that the transmitter 101 outputs differential signals.
Here, a timing chart of the operation of the transmitter 101 is shown in FIG. 10; operation of the transmitter 101 is explained below. As shown in FIG. 10, the control signal CTRL1 goes to high level and control signal CTRL2 goes to low level at time T11. As a result, transistors Tr1 and Tr2 become non-conducting, and so the output terminals CANH and CANL each go to an intermediate level. In this case, data received by the receiver 102 is recessive, indicating for example the low-level state.
On the other hand, at time T12 the control signal CTRL1 goes to low level and the control signal CTRL2 goes to high level. As a result, transistors Tr1 and Tr2 enter the conducting state, and current flows from transistor Tr1 to transistor Tr2 via the load resistance RL. Due to the flow of current in the load resistance RL, output terminal CANH goes to high level and output terminal CANL goes to low level. When output terminal CANH is at higher voltage than output terminal CANL, data received by receiver 102 is dominant, indicating for example the high-level state.
It is widely known that in an automobile in which a CAN is installed, transmitted and received data signals are substantially affected by external noise. In order to enhance the reliability of data signals with respect to such external noise, data is transmitted and received over a pair of wires in a CAN. Here, operation of a transmitter when external noise is intermixed in lines is explained, referring to FIG. 10.
First, a case is explained in which, at time T13 with data signals recessive, external noise is intermixed. At time T13′, external noise is intermixed equally in the line connected to output terminal CANH and in the line connected to output terminal CANL. At this time, transistors Tr1 and Tr2 are in the conducting state, so that there are approximately the same changes in potential in the output terminals CANH and CANL. Data received by receiver 102 is the potential difference between the output terminals CANH and CANL. Hence if there is no change in this potential difference, the data received by receiver 102 is not affected by external noise.
Next, a case is explained in which, at time T14 with output signals dominant, external noise is intermixed. At time T14′ external noise appears in the same way in the line connected to output terminal CANH and in the line connected to output terminal CANL. At this time, transistors Tr1 and Tr2 are in the non-conducting state, so that approximately the same potential changes occur in the output terminals CANH and CANL. Data received by receiver 102 is the potential difference between the output terminals CANH and CANL. Hence if there is no change in this potential difference, the data received by receiver 102 is not affected by external noise.
On the other hand, the transmitter 101 uses diodes as reverse-current prevention elements. Diodes have a charge-storage effect in which current flows in the opposite direction for a fixed period after switching from the conducting state to the non-conducting state, due to the discharge of accumulated charge. Consequently, if external noise is intermixed such that the potential at an output terminal is higher than the power supply voltage during the occurrence of the charge storage effect at a diode, a current flowing into the transmitter 101 occurs due to the intermixing of external noise. That is, when external noise having a large positive-side amplitude is intermixed, the charge storage effect occurs at diode D1, and current flows into the transmitter 101 from output terminal CANH. The inflowing current includes current which flows in from the line connected to output terminal CANH (path A′), and current which flows in via the loading resistance RL from the line connected to output terminal CANL (path B′). At this time, when passing through path B′, current flows in the load resistance RL. As a result, a potential difference occurs between output terminal CANH and output terminal CANL due to the current flowing in the load resistance RL and the load resistance RL. Due to this potential difference, signal noise occurs in data signals (time T15 in FIG. 10). In the case shown in FIG. 10, signal noise occurs below the recessive level which is the level of data signals after time T15.
The intrusion of external noise and current flowing due to the charge storage effect flow in the power supply terminal VDD via the parasitic diode formed between the drain and well of transistor Tr1 (paths A′, B′ in FIG. 9). Cross-sectional views of the PMOS transistor Tr1 and NMOS transistor Tr2 and of parasitic diodes appear in FIG. 11. As shown in FIG. 11, a parasitic diode is formed between the drain region 43 and the well region 42 in the PMOS transistor, and is in the conducting state when the potential difference between the drain potential and the power supply voltage exceeds the threshold of the parasitic diode. A parasitic diode is also formed between the drain region 47 and the well region 41 in the NMOS transistor, and is in the conducting state when the potential difference between the drain potential and the ground potential exceeds the parasitic diode threshold.
In an example of the related art, signal noise occurring in data signals attains a level greater than the difference between the recessive level and the threshold level or than the difference between the dominant level and the threshold level. This does not result in erroneous data recognition by the receiver 102. Hence even when signal noise occurs in data signals in a CAN due to a diode charge storage effect, there is no effect on data transmission and reception. Thus in a CAN which uses a pair of signal lines to transmit and receive signals using the potential differences of differential signals, the reliability of data signals is enhanced.
On the other hand, the FlexRay standard, which faster data communication speeds than in ordinary CAN, is currently being studied as a next-generation CAN standard. A CAN is a standard for event-triggered communication, in which the various circuits operate based on transmitted and received commands; FlexRay is a standard for time-triggered communication, in which the various circuits execute commands with prescribed timing. Hence whereas in a CAN there is no need to create a state with no data while data is not being transmitted and received, in a FlexRay system, states with no data must be created while data is not being transmitted or received. Consequently, whereas in a CAN data signals are transmitted and received using the two values of dominant and recessive, in a FlexRay system data signals are communicated using three values, which are for example Data1 indicating high level, Data0 indicating low level, and Idle indicating an intermediate level. Here, Idle is used as a value indicating a state with no data.
In FlexRay systems also, transmitters using reverse-current prevention elements such as diodes in bus driving circuits are generally employed. Transmitters used in FlexRay systems create an Idle state by not passing current in the bus driving circuit. Hence when making a transition from a Data1 or Data0 state to the Idle state, if external noise is intermixed while the charge storage effect is occurring in the diode, signal noise occurs in data signals.
In a CAN system, as explained above, the effects of this signal noise are not a problem. However, in a FlexRay system, Idle is an intermediate voltage, and thresholds higher and lower than this are provided, with Data1 at or above the higher threshold and Data0 at or below the lower threshold, so that when the Idle signal level exceeds a threshold due to this noise, there is the problem that the receiver may erroneously recognize the Idle state as Data1 or as Data0.
Hence in a transmitter which transmits three-valued signals such as in the case of FlexRay, even when diodes or similar are used as transmitter reverse-current prevention elements, it is desirable that measures be taken to prevent erroneous recognition of signals by the receiver.